1. Field of the Invention
This invention relates to a retouching agent for lithographic printing plate and, more particularly, to an agent that can be used to erase a portion of the image area of a lithographic printing plate prepared by a photochemical process or an area that repels the fountain solutions (or dampening water) used in lithographic printing and receives oily ink.
2. Description of the Prior Art
Japanese Patent Publication No. 33442/76 (corresponding to British Pat. No. 1,408,709) teaches a retouching agent that contains a lactone having 3 to 6 carbon atoms and which is applied to a lithographic printing plate prepared from a photosensitive material in which the photosensitive layer contains an o-quinonediazide compound (this plate being referred to as a presensitized plate and dubbed a PS plate in the art). Japanese Patent Publication No. 16047/71 (corresponding to British Pat. No. 1,272,868) teaches a correcting medium for a photomechanically prepared lithographic printing plate that contains:
(1) 20 to 60 wt% of glycol, polyglycol, glycol ether or polyglycol ether; PA1 (2) 10 to 50 wt% of a neutral organic solvent other than those mentioned under (1); PA1 (3) 3 to 25 wt% of water; PA1 (4) 0.5 to 15 wt% of acid or a substance exhibiting acidity in an aqueous solution; and PA1 (5) 5 to 25 wt% of a thickening agent, the liquids mentioned under (1), (2) and (3) being so selected and in such proportion that they are completely miscible.
Conventional retouching or correcting agents have the following defects. The vapors of the solvent used in the retouching agent disclosed in Japanese Patent Publication No. 33442/76 act on a portion of the image areas adjacent those to be corrected. The agent also has the disadvantage of slow erasing speed. The correcting medium described in Japanese Patent Publication No. 16047/71 requires the use of a fluorine-containing compound. This compound not only attacks a brush and like applicators used in applying the medium to a particular portion of the image area of the lithographic plate but it is also hazardous and presents pollution problems.